Membrane Properties

Pore size is the most important property of a UF membrane. Pores can be visualized using electron microscopy. Surface porosity (the proportion of the membrane surface occupied by pores) is less than 10% for many UF membranes. In an ideal membrane, all pores should be of the same size. In reality, there is a distribution of pore sizes, as shown in Figure 3. This makes it difficult to get a sharp separation of similarly sized molecules by UF. A common method to characterize UF membranes is to challenge the membrane with several macromolecules of known molecular sizes. Since proteins of different molecular weights are usually used as molecular markers, UF membranes are characterized in terms of their ability to retain proteins of a particular molecular weight. Figure 3 is a graphical representation of solute rejection data for ideal and real membranes. No membrane will display the sharp pore-size distribution shown for the ideal membrane. MF membranes are given 'absolute' ratings which is the largest particle that will be retained by the membrane, based on actual tests under standard conditions. In contrast, UF membranes are given 'nominal' ratings which refer to the molecular weight of a test solute (ideally it should be a globular protein) which is 90% rejected by the membrane under standard conditions. This rating is termed the molecular weight cut-off (MWCO) of the membrane.

Proteins are not ideal compounds to use for this purpose, since their molecular size can be affected by pH, ionic strength and interactions with buffer components. Proteins can have different isoelectric points, solubility and hydrophobicity, thus causing them to interact with and foul the membrane to different extents, which affects measured rejections. In addition, proteins which differ by 10 times in MW may only differ by three times in size in their globular form. Owing to the difficulty of finding proteins that are sufficiently pure (and inexpensive) to conduct MWCO evaluations, other compounds such as polyethylene glycols (PEG) and dextrans have been used because they are water soluble and can be readily obtained with well-defined and narrow-size distributions. Since the shapes of these various compounds are different, the MWCO profile of a membrane will also differ depending on the solute test marker used. Environmental conditions such as pH and ionic strength also affect shape and conformation of molecules which can affect rejection.

Other components in the feed solution could affect the separation of the target compound. For example, with UF membranes, low-molecular-weight solutes (such as sugars and salts) have molecular sizes much smaller than the smallest pore on the membrane. These compounds will be freely permeable, i.e. they will have zero rejection, unless they interact with or bind to impermeable compounds in the feed. Changes in operating conditions will not affect their permeability. On the other hand, large solutes that are much bigger than the pores will be completely rejected (i.e. 100% rejection). Its rejection properties will also be relatively unaffected by operating conditions or if other compounds are present. However, if the solute has a size that is of the same order of magnitude as the pore, its rejection may be affected in the presence of the large molecule. This is because the large molecule forms a secondary

Figure 3 Typical molecular-weight profile of ideal and real membranes. Relationship shown is between molecular size of a solute in the feed stream and rejection of the solute by the membrane. (Adapted from Cheryan (1998) with permission from Technomic.)

dynamic membrane on the original membrane that inhibits passage of the smaller molecule. Operating conditions that change the shape or conformation of the solute will also affect its rejection.

As a general rule, fractionation of polymers can be accomplished if there is at least a 10-fold difference in molecular weight. Separation of similarly sized mac-romolecules can be enhanced by diluting the feed to minimize solute-solute interactions and solutemembrane interactions.

Other factors affecting separation are operating parameters such as pressure and cross-flow rate. These control the degree of turbulence and the thickness of the boundary layer and extent of concentration polarization (defined below), which in turn affect permeability of smaller solutes.

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